Electromagnetic (“EM”) telemetry may be used to transmit data from a downhole tool in a wellbore to a receiver at the surface. EM telemetry may be bi-directional with half-duplex transmitters and receivers. EM telemetry may implement a time-sharing schedule between uplink and downlink commands Real-time (“RT”) data transmission allows for real-time interpretation and decision-making that may be used for steering, well placement, drilling optimization, and safety. While most downhole tools have high-resolution versions of their data in memory (called RM—Recorded Mode), in many cases the RM data may be too large to transmit to the receiver at the surface in real-time.
When there is data corruption (e.g., due to noise burst, signal loss, or other factors), it may be desirable to be able to fetch previous RT data/log sections that were corrupted. Further, it may be desirable to improve the quality of the RT data/log sections previously received. Conventional downhole tools are aware of the measurements that have been taken and the nature of those measurements. Conventional downhole tools are not, however, aware of telemetry channel conditions. The receiver at the surface is aware of the condition of the received signal subjected to the telemetry channel, including whether it was able to decode the transmitted data. However, the receiver at the surface is not aware of the measurements downhole unless the measurements are successfully received and decoded at the surface. If the same information was available to both the downhole tool and the receiver at the surface, then it would be possible to determine the data block that is transmitted, the time of transmission, and the bit rate.
In EM telemetry, mud pulse telemetry, acoustic telemetry, and wired drill pipe telemetry, the transmission rate of the channel is well below the data entropy/acquisition rate. Thus, to support a real-time communication and maintain a minimal latency, the data is compressed prior to transmission. For bi-directional communication, data compression algorithms that output a progressive bit stream may be used.
Traditional compression algorithms, such as JPEG and H.264/AVC, are often optimized for one particular bit-rate/quality level. Once the data is compressed, there is little freedom in changing the bit rate/quality attributes. In fact, the data is re-compressed to meet new target specifications. This poses two main challenges for bi-directional communication. First, after the downlink is received, the downhole tool then accesses the recorded memory and re-compresses the data to a new target transmission rate. The compression of multiple blocks of data can increase the algorithm complexity. Second, in bi-directional communication, a low quality version of the data may already be available at the surface. With conventional compression methods, there is no trivial way to re-use this information. If the user wishes to improve the quality of a block, the downhole tool compresses the data without any knowledge of the available data at the surface.